1. Field of the Invention
The present invention relates to downhole sliding sleeve valves. More particularly, the present invention relates to such a sliding sleeve valve having a wear-resistant seal ring.
2. Description of the Prior Art
Sliding sleeve valves are used in numerous downhole applications in the oil and gas industry. Examples of such valves are sliding sleeve valves sold under the trademark "SLIDING SIDE DOOR" type XA by Otis Engineering Corp. In particular, U.S. Pat. No. 5,263,683 ('683 Patent), incorporated herein by reference for all purposes, discloses a sliding sleeve valve or sliding sleeve of the type under consideration. As disclosed in the '683 Patent, the sleeve valves have a tubular housing or main body that can comprise one or more assembled portions and that can be made up into a string of well conduit (typically production tubing, but conceivably drill pipe or some other conduit type) as part thereof. The valve can be used to selectively prevent or permit flow between the well annulus and the interior of the string of conduit, e.g., a tubing string. For example, packers in the string above and/or below the valve can be used to pack off or isolate a given zone of the wellbore. The sleeve can be left closed to maintain that isolation or, when it is desired to produce from that zone, the sleeve can be opened to permit fluid to flow from the zone into the tubing string. In other cases, the valve may be opened to permit a fluid to pass from the interior of the tubing string into the annulus.
In a sleeve valve such as disclosed in the '683 Patent, one or more flow ports extend radially through the housing wall. A valve element in the form of a tubular sleeve carried coaxially within the housing is provided with and is movable longitudinally relative to the housing, from a first, or closed, position in which the sleeve blocks off flow through the flow ports, prohibiting flow between the annulus and the interior of the valve and a second, or open, position wherein the flow apertures in the sleeve are in register with the flow ports in the housing so that fluid can flow from the annulus into the tubing string or vice versa. In such sleeve valves, there are a series of upper and lower auxiliary seal stacks to effect dynamic sealing between the housing and the sleeve, which passes across the seals during opening and closing of the flow port through the housing.
It frequently happens that there is considerable pressure in the annulus surrounding the housing, thereby creating a large pressure differential between the annulus and the interior of the valve. Accordingly, during movement of the sleeve to open the flow ports to permit fluid communication between the interior and exterior of the valve, the auxiliary seals positioned between the housing and the sleeve will first be exposed to a surge of fluid flow, which can cause extensive damage to the seals as pressure is equalized before a full positive opening of the sleeves and, in some instances, during complete opening of the sleeve. In any event, any time such auxiliary seals are exposed to flow surging, since they are dynamic in nature, a leak path could be formed through the auxiliary seals. One way to at least partially prevent damage to the auxiliary seal stacks is to dissipate or equalize this large differential pressure, as by allowing slow bleeding of the high pressure fluid from the annulus into the valve before full production flow is established.
One way to accomplish this pressure equalization, and as disclosed in the '683 Patent, is to provide the sleeve element with a small pressure relief orifice, which, when the valve is in the closed position, is disposed between a primary seal, which seals between the housing and the sleeve, and the flow apertures in the sleeve and is designed to protect the auxiliary seal stack, the primary seal being positioned between the auxiliary seal stack and the flow port. Accordingly, the sleeve element can be moved longitudinally within the housing in a first direction until the pressure relief orifices are in open communication with the flow ports in the housing to allow fluid to bleed from the annulus slowly through the pressure relief orifices until the pressure in the tubing string is approximately equal to that in the annulus. Following the pressure equalization, the sleeve element can be moved still further in the first direction to an open position, wherein the flow ports in the housing and the flow apertures in the sleeve are in register, providing substantially greater flow area than the pressure relief orifices. Obviously, it would be desirable if the pressure equalization step could be eliminated such that the valve could be moved from the closed position directly to the full open position.
When the sleeve element is moved from the closed position to the equalizing position, the pressure relief orifices slide across the primary seal. The large pressure differential between the annulus and the interior of the valve acts on the primary seal, urging it radially inwardly against the sleeve element. Thus, when the edges of the pressure relief orifice slide past the primary seal, there is a tendency to clip off a bit of the material from which the primary seal is made, eventually ruining its sealing effectiveness.
In the '683 Patent, it was found that this clipping of the primary seal could be largely eliminated by forming recesses in the outer surface of the sleeve in surrounding relationship to the orifice, the recesses having a dimension that is longer, in the axial direction, than the radially outer end of the pressure relief orifice. The recess is sized, configured, and positioned to permit the pressure relief orifice to pass the primary seal while moving from the closed position to the pressure relief position without substantial damage to the primary seal, even when the latter is made of an elastomeric material, e.g. an O-ring.
Even though the sleeve valve of the '683 Patent overcomes the seal clipping problems to a substantial extent, it has been found that under severe operating conditions, e.g., elevated temperatures, and/or conditions where the primary seal is subject to chemical attack, the primary seal still undergoes an undesirable failure rate. Thus, it would be desirable if the sleeve valve possessed a primary seal assembly or member that would form an effective seal between the housing and the sleeve and would permit the valve to go from closed to full open without equalization while still protecting the auxiliary seal stack.